evaluating sdbd. crown & bridge downbearings in a new piano

[Delwin D Fandrich]

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  ----- Original Message -----=20
  From: Frank Weston=20
  To: pianotech@ptg.org=20
  Sent: Tuesday, September 28, 1999 5:46 AM
  Subject: Re: evaluating sdbd. crown & bridge downbearings in a new =
piano


  Richard,

  .....

  Here is an experiment I conducted in my shop yesterday.  I selected a =
room dried strip of sitka spruce, made pencil marks on it 18 inches from =
the end.  I bent the spruce to an approximate 3' radius and glued a rib =
to the bottom.  When I measured the top of the spruce, I found that the =
pencil mark position indicated the top had stretched approximately 1/16 =
inch, and the bottom had compressed about as much.  This model indicates =
to me that the top is in tension and the bottom in compression. =20

  I then placed the model between constraints at each end and loaded the =
top.  As the load increased, the line on the spruce showed me that =
tension in the top was decreasing.  I loaded the model to failure.  The =
rib failed first, and the failure was the most beautiful textbook =
indication of a failure in tension on the top and compression on the =
bottom (if you don't know what this kind of failure looks like, buy a =
box of chalk and break a few pieces in different ways - the most =
interesting is when you break it by twisting).  Because of one of =
Newton's laws, we might deduce that the soundboard was experiencing =
equal and opposite loads when the rib failed.
  =20
  If you care to construct such a model, I'm sure your results would be =
similar.  If you would like to see pictures, I will consider putting =
them up on a photo site.  If the actual physical demonstration of the =
main point I am arguing does not convince, then I fear we must promote =
this discussion to the level of religion where issues are resolved on =
the basis of faith and personality.
  =20
  Frank Weston

  -----------------------------------------------

  Good grief, man, the top piece did not "stretch," you wrapped it =
around a larger circle!=20

  I am working on a similar experiment.  The results and conclusions =
will be posted later.

  In the meantime, if you really want to understand this subject, I =
would suggest that you take the results of your experiment and discuss =
them with a local engineer.  Perhaps he/she can think of some example =
that will convince you of the error of your ways.

  Regards,

  Del

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